This invention relates to a method of making overclad fiber optic couplers that exhibit low excess loss.
Fiber optic couplers referred to as "fused fiber couplers" have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Various coupler properties can be improved by encapsulating the coupling region of the fibers in a matrix glass to form an "overclad coupler". Portions of the fibers to be fused are inserted into a glass tube having a refractive index lower than that of the fiber claddings. The tube has a longitudinal bore, each end of which is preferably connected to a respective tube end surface by a funnel that facilitates the insertion of the fibers. Since optical fibers have a protective coating thereon, that portion of the fibers that is to be positioned in the tube midregion must have the coating stripped therefrom. After the fibers are stripped and the uncoated portions are positioned in the tube bore, the tube midregion is collapsed onto the fibers; the central portion of the midregion is then drawn down or stretched to achieve the desired coupling.
In certain types of couplers, coupling is increased by twisting the fibers along the fused length of the fibers (U.S. Pat. No. 4,426,215). However, twisting the fibers is disadvantageous for certain purposes. Indeed, even microbends in the fiber can increase coupler excess loss. Therefore, various techniques have been developed to prevent or reduce fiber twisting or bending in overclad fiber optic couplers.
U.S. Pat. No. 4,931,076 teaches that the length of each fiber from which coating has been stripped should be shorter than the length of the tube bore to allow the coating to extend into both ends of the bore, thereby positioning each fiber within the bore cross-section. That patent further teaches that the lengths of the uncoated portions should be about equal and that those uncoated portions should be adjacent one another.
U.S. Pat. No. 5,009,692 also teaches the placement of fiber coatings in the tube bore, and further suggests that holes that are diamond-shaped or the like prevent the rotation of the fibers during the tube collapse step. That patent further states that regardless of the geometry of the bore, it is preferred that the bore size be no larger than that size that is sufficient to receive the coated fibers without causing the coating material to smear onto the wall of the bore, excessive bore size causing macrobend loss and/or twisting of fibers. It is noted that when the coatings of both fibers of a 2.times.2 coupler are situated in the tube bore, the bore diameter must be at least twice the diameter of the fiber coatings.
Couplers have also been made by stripping coating from the central portion of each of the the fibers, all uncoated portions being longer than the tube bore. Thus, when the uncoated portions of the fibers are centered in the tube bore, the coated portions of the fibers end in the middle of the funnels. This technique can employ a smaller bore diameter than the methods of the aforementioned U.S. Pat. Nos. 4,931,076 and 5,009,692; however, the fibers are not tightly held in parallel alignment during the tube collapse step.